Hermetic electrically insulating materials are desirable for packaging of electronics in implantable stimulating and sensing devices, where implantation is in living tissue in a living body. The implanted devices must be biocompatible. The devices must not only exhibit the ability to resist the aggressive environment present in the body, but must also be compatible with both the living tissue and with the other materials of construction for the device itself. The materials are selected to avoid both galvanic and electrolytic corrosion. Typical materials of construction for implantable devices include ceramics, plastics, or metals. The ceramics may be glass, or a metal oxide, such as alumina, titania, zirconia, stabilized-zirconia, partially-stabilized zirconia, tetragonal zirconia, magnesia-stabilized zirconia, ceria-stabilized zirconia, yttria-stabilized zirconia, or calcia-stabilized zirconia, or yttria-stabilized zirconia, although other ceramic materials may also be used. The plastics may be epoxy, polycarbonate, or plexiglass. Typical metals include titanium or titanium alloy (such as Ti-6 Al-4 V), although other metals, such as platinum, iridium, platinum-iridium, stainless steel, tantalum, niobium, or zirconium may be used.
One solution to achieving biocompatibility, hermeticity, and galvanic and electrolytic compatibility for an implanted device is to encase the device in a protective environment. It is well known to encase implantable devices with glass or with a case of ceramic or metal. Schulman, et al. (U.S. Pat. No. 5,750,926) is one example of this technique. It is also known to use alumina as a case material for an implanted device as disclosed in U.S. Pat. No. 4,991,582. These cases are often too thick for use with miniature implantable devices, such as the prosthetic retinal implants of Second Sight, LLP. The case unacceptably increases the size of the device and becomes a limiting factor as to where the devices may be placed in the body.
It is also known to protect an implantable device with a thin layer or layers of an electrically insulating, protective material, as disclosed by Schulman, et al. (U.S. Pat. No. 6,043,437). Coatings of alumina, zirconia, or other ceramic, less than 25 microns thick, were applied by evaporative coating, vapor deposition, or ion-beam deposition.
Disadvantageously, the sensor described in the referenced patent and patent applications is relatively thick. For some applications, where small size is required, such as when a device is placed in an eye, eyelid, or in a fingertip, space is very limited and only a very small device will fit. There remains a need for yet a smaller sensor or a stimulator that performs all of the same functions as the prior apparatus, i.e., that provides working electrodes exposed to living tissue, perhaps with a selected enzyme placed over one electrode, and with hermetically-sealed electronic circuitry controlling the stimulator or sensor and communicating with other internal or external devices. The present invention advantageously addresses these and other needs.
U.S. Pat. No. 5,660,163 discloses an implantable glucose sensor that is fabricated on a ceramic substrate. Working electrodes and other elements associated with the sensor are exposed to a conductive fluid contained within a reservoir or inner sheath that covers the substrate. An outer sheath is also placed over the sensor, with a window formed over one of the working electrodes. A selected enzyme, such as glucose oxidate, is placed within the window. As disclosed in U.S. Pat. No. 5,660,163, five wires or conductors are attached to the electrodes and connected to electronic circuitry, e.g., a circuit such as is shown in FIG. 3 of the '163. U.S. Pat. No. 5,660,163 is incorporated herein by reference.
Additional features, aspects, and improvements of a glucose sensor of the type disclosed in U.S. Pat. No. 5,660,163 are further disclosed in U.S. Pat. No. 6,081,736; U.S. Pat. No. 6,119,028; and U.S. Pat. No. 5,999,848; each of which above-referenced patent applications is incorporated herein by reference.